1)   Hemagglutinin (HA) at wikipedia

2)  Activation of Influenza A is achieved by cleavage of the hemagglutinin (HA) to HA1 and HA2.  This cleavage is carried out by host proteases at a well defined cleavage site, usually consisting of a single arginine (monobasic).

Have a look at the structure of flu HA with this JMol demo (on a Mac, use Safari)

The monobasic cleavage site, usually present in influenza A, is recognized and cleaved by proteases, such as tryptase Clara, a trypsin-like protease secreted from specialized cells in the bronchial endothelium.  Thus the tissue tropism of influenza is limited, to some extent, by the presence of appropriate host proteases. 
In contrast, several highly pathogenic avian HA subtypes have an insertion mutation creating a multi-basic cleavage site with an -R-X-R/K-R- motif. A multi-basic HA cleavage site allows the HA protein to be activated by other more ubiquitous proteases such as Furin (4).  The implication of this cleavage site mutation is that the virus can spread to, and replicate in, tissues not normally affected by influenza.

These multi-basic motifs were found only in avian H5 and H7 influenza thought not to infect humans until the 1997 Hong Kong outbreak in which 16 people were infected with an avian H5N1 influenza virus (4, 5). 

Comparison of the H5N1 Hong Kong strain with sequences from the currently circulating swine H1N1 HA gene (shown below) show a 4 amino acid insertion in the cleavage site of the H5N1 HA gene creating a multi-basic site, providing 1) a mechanism for this virus to display pantropism and 2) a possible explanation of the differences in virulence.

Alignment created with Base-By-Base at www.virology.ca